Method for removing cesium from a nuclear reactor coolant

ABSTRACT

A method of and system for removing cesium from a liquid metal reactor coolant including a carbon packing trap in the primary coolant system for absorbing a major portion of the radioactive cesium from the coolant flowing therethrough at a reduced temperature. A regeneration subloop system having a secondary carbon packing trap is selectively connected to the primary system for isolating the main trap therefrom and connecting it to the regeneration system. Increasing the temperature of the sodium flowing through the primary trap diffuses a portion of the cesium inventory thereof further into the carbon matrix while simultaneously redispersing a portion into the regeneration system for absorption at a reduced temperature by the secondary trap.

The United States Government has rights in this invention pursuant toContract No. EY-76-C-14-2170 between the U.S. Department of Energy andthe Westinghouse Electric Corporation.

BACKGROUND OF THE INVENTION

The present invention relates generally to nuclear reactors and, moreparticularly, to the removal of cesium from the liquid metal coolant ofa fast breeder nuclear reactor.

A typical fast breeder nuclear reactor employs liquid sodium as acoolant to remove the tremendous heat generated by the nuclear fissionof fissile materials. The heat carried by the coolant is ultimatelytransformed into steam via a secondary system for the generation ofelectrical energy. The liquid sodium is circulated through a closed heattransport system known as the primary coolant system which includes thereactor vessel, a heat exchanger or a steam generator, a suitable pipingsystem for serially connecting these components together, and a pump forcirculating coolant therethrough. The liquid sodium can be substantiallycontaminated by the volatile, radioactive fission products resultingfrom breached or vented fuel elements during reactor operation. Cesiumis known to be one of the dominant radioactive contaminants found in theprimary coolant system and its deposition on the cooler surfaces ofsodium coolant systems has often been observed. The presence of cesiumin the primary coolant system poses a safety and health hazard,especially during reactor refueling, maintenance and/or primary coolantsystem repairs wherein contact maintenance is required with resultantpersonnel exposure to the hostile environment. Also, any leakage of thecontaminated coolant from this primary system, although highlyimprobable, would pose further safety risks. Accordingly, it can beappreciated that cesium removal is of paramount importance in reducingthe health hazards associated with fast breeder nuclear reactors.

Various attempts have been made to solve this cesium contaminationproblem including the use of a cold trap purification system. However,this arrangement has been only marginally effective in removing cesiumand the extent or magnitude of cesium removal has varied widely. Thisvariability in cesium deposition behavior is believed due tointeractions with other contaminants in the sodium system with bothoxides and hydrides being proposed as possible contaminants responsiblefor increased cesium deposition. However, other approaches involvingdeliberate additions of oxygen and hydrogen, respectively, suggestedthat oxide and hydrides do not enhance cesium deposition. One of themore successful solutions in removing cesium from a sodium coolantsystem involves the use of a special carbon packing trap provided in thecirculating system of the liquid sodium flow. Such a trap was employedin the sodium coolant system of the Experimental Breeder Reactor II(EBR-II) facility in Idaho Falls, Id. It was found that under relativelylow temperature conditions, graphite or amorphous carbon packingemployed in the trap could remove about 90% of the cesium activity fromthe sodium in the primary coolant system.

The present invention constitutes an improvement over this cesiumremoval system and is directed to a method and system for regeneratingthe primary carbon packing trap of the primary system to realize furtherreductions in the cesuim activity therein than has heretofore beenpossible.

Accordingly, it is a primary object of the present invention to providea new and useful method and system for more efficiently removing cesiumfrom a reactor coolant.

It is another object of this invention to provide in the primary coolantsystem of a nuclear reactor a regeneration flow system selectivelyconnected to the primary system's main trap for reducing the cesiuminventory otherwise entrained therein.

It is a further object of the present invention to provide a new anduseful regeneration method and system for regenerating the main cesiumtrap in a sodium coolant system and improving its cesium absorptioncapabilities.

These and other objects, advantages, and characterizing features of thepresent invention will become clearly apparent from the ensuing detaileddescription of an illustrative embodiment thereof, taken together withthe accompanying drawings wherein like reference characters denote likeparts throughout the various views.

SUMMARY OF THE INVENTION

The present invention is characterized by the provision of aregeneration subloop system selectively connected to a primary liquidmetal coolant system having a primary carbon packing trap for absorbingcesuim from the coolant flowing therethrough at a relatively lowtemperature. When saturated with cesuim, the primary trap is isolatedfrom the primary system and connected to the regeneration system whichincludes a secondary carbon packing trap. The coolant passing throughthe primary trap is heated to facilitate dispersion of a portion of thecesuim inventory therof into the substantially smaller coolant volume ofthe regeneration system. This highly contaminated coolant is reduced intemperature prior to flowing through the secondary trap to enhancecesuim absorption thereby.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a portion of a nuclear reactorprimary coolant system incoporating one form of a regeneration subloopsystem of this invention;

FIG. 2 is an elevational view, partly broken away and in section, of atrap utilized in the present invention; and

FIG. 3 is another form of a regeneration subloop system embodying theprinciples of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the illustrative embodiment depicted in theaccompanying drawings, there is schematically shown in FIG. 1 a portionof a primary coolant system, comprehensively designated 10, whichincludes a filtering arrangement for removing cesium from the reactorcoolant and which embodies certain novel features of this invention.This system 10 includes a main coolant flow conduit 11, forming a partof the primary coolant system or otherwise coupled in parallel thereto,preferably in the cold leg of the system downstream of the usual heatexchanger (not shown). As is well known, the primary reactor coolant,such as liquid sodium, is heated to extreme temperatures on passagethrough the core of a nuclear reactor pressure vessel to remove heattherefrom. This hot liquid sodium flows through the primary system to aheat exchanger for transferring heat from the primary system to anotheror secondary coolant system coupled in sealing arrangement with theprimary system for ultimate conversion into steam in order to generateelectrical energy.

Sometimes, an intermediate system is interposed between the primary andsecondary systems of the nuclear reactor for safety reasons. Thisintermediate system also is closed and contains a liguid metal coolant,such as liquid sodium for example, while the secondary system enploysthe necessary water for conversion into steam.

The sodium coolant flowing through conduit 11, previously reduced intemperature by passage through the heat exchanger, is further reduced intemperature by cooler 12 to approximately 200° C. This cooled sodium isconveyed through a normally open shut-off valve 13 and routed through aspecially constructed filter or trap, generally designated 15, forremoving cesium from the flowing liquid sodium. The treated or cleansedliquid sodium exiting trap 15 is routed via conduit 11a through anormally open shut-off valve 16 oack to the primary coolant system.

An exemplary form of cesium trap 15 is illustrated in FIG. 2 and isenclosed in a containment 17 filled with a suitable thermal insulatingmaterial 18. The trap 15 comprises a generally cylindrical housing orshell 20 having a domed top wall 21, a bottom wall 22 and an annularside wall 23. The shell 20 is supported vertically on a suitable base orpedestal 25 within containment 17. The upper end of the shell 20 is inopen communication with the outlet end 26 of a conduit 27 suitablyconnected to the conduit 11 at the entry end of trap 15 for receivingthe contaminated liquid sodium coolant.

A vertically extending inner tube section 30 is mounted centrally withinthe shell 20 in radially spaced relation thereto. The upper end of innertube section 30 is connected to the inlet end 31 of a conduit 32suitably connected to the sodium conduit 11a at the exit end of trap 15for returning the cleansed or treated sodium back to the primary coolantsystem.

The trap 15 is substantially filled with a packing 33 of low densityamorphous carbon foam, preferably reticulated vitreous carbon, whichserves to remove or absorb cesium from the liquid sodium passingtherethrough. The reticulated vitreous carbon packing 33 provides alarge surface area per unit volume and facilitates uniform flowdistribution over the packing surface.

In order to prevent the escape of any packing particulates which tend tofragment during operation, a 35-micron sintered stainless steel screen34 is provided for retaining the packing 33 within shell 20. The screen34 is mounted centrally of shell 20 between the bottom end of tubesection 30 and a mounting plate 35. The screen 34 is formed with an opencentral passage 36 communicating with the tube section 30. An annularperforated plate 37 is disposed on an upper surface of the packing 33near the upper end of shell 20 and serves to maintain the packing 33intact. Thus, the incoming liquid sodium is passed through the carbonpacking 33 for removing the major portion of cesium activity therefrom.The cleansed or treated sodium flows through the screen 34 inwardly intopassage 36, and via tube section 30, inlet 31 and conduit 32, isdelivered into conduit 11a for reentry into tne primary coolant system.

It should be understood that the use of carbon to control cesiumcontamination has been previously proposed. It was found that cesiumcould be appreciably removed from liquid sodium by graphite or amorphouscarbon packing at relatively low temperatures ranging from about 170° C.to about 230° C., and preferably about 200° C. The cesium is not merelyphysically adsorbed on the packing surface but is chemically bonded tothe carbon. Such carbon packings form a number of compounds with cesiumwithout in any way affecting the sodium. However, the reducedtemperatures which were found to favor cesium activity absorption by thecarbon packing are too low to permit adequate diffusion of the cesiuminto the carbon matrix at a useful rate. Accordingly, only the packingmaterial in the immediate vicinity of the packing surface is utilizedduring these low temperature trap operations so that the packingsaturates after removing approximately 90% of the radioactive cesiumfrom the liquid sodium. While raising the trap temperature substantiallyabove the hereinbefore mentioned range would be desirable in diffusingthe cesium further into the carbon matrix, it has the disadvantage ofshifting the equilibrium toward redispersing the cesium back into theliquid sodium. Thus, while the known carbon filtering arrangement so fardescribed has admirably served its purpose in removing a major portionof radioactive cesium from a liquid sodium coolant, its cesium removalcapability remains limited.

The present invention addresses this problem and increases theefficiency of the above described cesium removal system by providing anauxilliary or subloop system, generally designated 40 (FIG. 1), forregenerating the main or primary trap 15 as will hereinafter bedescribed. This regeneration system 40 includes a conduit 41 having aninlet 42 tapped into the main conduit 11 between the valve 13 and theinlet of primary trap 15 and an outlet 43 tapped into conduit 11abetween the outlet of primary trap 15 and valve 16. Flow through theconduit 41 is selectively controlled by an inlet shut-off valve 45 andan outlet shut-off valve 46. The conduit 41 directs sodium flow througha cooler 47, a pump 48 for circulating the liquid sodium in the isolatedsubloop system 40, a secondary or regeneration trap 50, and a heater 51downstream of the trap 50, all serially connected by the conduit 41.

The regeneration trap 50 can be identical in construction and operativein the same manner as primary trap 15, except that the former issubstantially smaller than trap 15, containing a packing volume of fromabout 10% to 50% of the primary trap packing volume for example. Itshould be appreciated that the regeneration trap 50 of this invention isnot limited to the specific trap described and shown in FIG. 2, but canvary widely in construction and configuration within the purview of thepresent invention, so long as it embodies a suitable carbon packingmaterial through which the contaminated coolant flows. Also, the totalvolume of sodium circulated in the regeneration system 40, which wouldinclude the primary trap 15 in operation, is approximately 10⁻³ to 10⁴of the volume of the primary coolant system.

In operation under normal conditions, the temperature of the sodiumflowing through main conduit 11, as reflected in the cold leg of theprimary coolant system, is a reduced to approximately 200° C. by cooler12 prior to passing through the primary trap 15, wherein the majorportions of the radio active cesium is removed from the sodium. At thistime, inlet and outlet valves 45 and 46 are closed to disconnect theregeneration system 40 from the primary system. As earlier noted, thefiltering or cesium absorption operation continues until the carbonpacking 33 in primary trap 15 reaches saturation upon removingapproximately 90% of the cesium activity from the primary sodium coolantsystem.

In accordance with this invention, after the primary trap 15 loses itscesium removal efficiency, it is isolated from the primary system byclosing normally open valves 13 and 16 and is connected to theregeneration system 40 by opening normally closed valves 45 and 46.Circulation is established in the regeneration subloop system 40 by pump48 and a differential temperature is created across the regenerationsystem 40 by activating cooler 47 and energizing the heater 51.Preferably, cooler 47 is effective to maintain the sodium passingthrough regeneration trap 50 at about 200° C. heater 51 increases thetemperature of the sodium passing through the primary trap 15 to about500° C. At this elevated temperature, the cesium inventory in theprimary trap 15 tends to disperse into the limited sodium volume ofsubloop system 40 while some of the cesium within packing 33 of trap 15will diffuse further into the packing matrix thereof. In the meantime,the carbon packing within the regeneration trap 50 at the reducedtemperature of about 200° C. now begins to absorb cesium from therelatively high concentration of cesium contamination in the drasticallyreduced sodium volume of the isolated subloop system 40. Thus, primarytrap 15 releases cesium while regeneration trap 50 absorbs oraccumulates cesium at these respective controlled temperatures. Thisactivity continues until equilibrium is reached at which time theprimary trap 15 is then isolated from the regeneration subloop system 40by closing valves 45 and 46 and reconnected to the primary system byopening valves 13 and 16. The primary trap is then cooled to its normaloperating temperature of about 200° C. and, because of the reconstitutedcarbon packing surface, i.e. the reduced cesium concentration therein,is now conditioned to effect a still further reduction of theradioactive cesium concentration in the primary coolant system. Thus, byregenerating the primary trap 15, appreciably more than 90% of thecesium contamination in the primary sodium coolant system can beremoved. Indeed, even further trapping efficiency can be realized byperiodically isolating the regeneration trap 50 and connecting the sameto still another regeneration subloop system for reducing its cesiuminventory and diffussing a portion thereof deeper into its carbonpacking. In the same manner described above, the secondary trap 50 canbe reconditioned to, in turn, further absorb more of the cesium contentof primary trap 15.

FIG. 3 illustrates another form of a regeneration subloop system 40'which differs primarily from the regeneration system 40 first describedby utilizing a heat exchanger 52 to minimize the direct heating andcooling requirements otherwise needed during regeneration. Also, theheater 51 is provided in the main conduit 11 rather than in the conduit41 of regeneration subloop system 40'. The same reference characters areemployed to identify components similar to the form of the inventionearlier described.

Under the usual operating conditions, valves 13 and 16 are open whileinlet and outlet valves 45 and 46 are closed to disconnect theregeneration system 40'. Also, heater 51 is deenergized. Thus, thetemperature of the sodium coolant flowing through conduit 11 is reducedin heat exchanger 52 and further reduced, if required, to approximately200+ by cooler 12 prior to entry into the primary trap 15. The treatedand cooled sodium then flows through the shell side of exchanger 52 inheat exchange relation to the hot sodium flowing through conduit 11. Thesodium exiting the shell side of exchanger 52 is then returned viaconduit 11a to the primary coolant system.

When the primary trap 15 becomes saturated as earlier described, it isisolated from the primary coolant system by closing valves 13 and 16 andconnected to the regeneration subloop system 40' by opening valves 45and 46. Flow is established in this subloop system 40' by pump 48 and adifferential temperature is created across the subloop 40' bydeactivating cooler 12 and activating heater 51 and cooler 47. Thus, aclosed system is provided in which the sodium flowing through conduit 11passes through the heat exchanger 52, is heated to approximately 500° C.by heater 51 and then flows primary trap 15. The sodium exiting trap 15passes through the shell side of heat exchanger 52 wherein it issomewhat reduced in temperature, is further reduced in temperature toabout 200° C. by cooler 47, and then passes through regeneration trap50. As in the first instance, some of the cesium inventory in primarytrap 15 will diffuse further into the carbon packing matrix thereofwhile some will be redispersed back into the limited sodium volume ofregeneration subloop system 40' and absorbed by regeneration trap 50until equilibrium is achieved. The primary trap 15 is then disconnectedfrom subloop system 40' by closing valves 45 and 46 and reconnected tothe primary coolant system by opening valves 13 and 16. Cooler 47 andheater 51 are then deactivated while cooler 12 is activated to againcool the hot sodium from the primary system to approximately 200° C. fortreatment through the reconditioned primary trap 15, the latter nowbeing effective to realize a greater than 90% reduction of the cesiumcontent in the primary sodium coolant system.

From the foregoing, it is apparent that the objects of the presentinvention have been fully accomplished. As a result of this invention, anew and improved system is provided for enhancing the removal of cesiumfrom a liquid sodium coolant with greater efficiencies than haveheretofore been realized. By the provision of a regeneration subloopsystem having a secondary cesium trap and which is selectively connectedto a primary sodium coolant system, the primary cesium trap can bereconditioned to materially extend its useful life while enhancingcesium removal from the primary sodium coolant system.

It is to be understood that the forms of the invention herein shown anddescribed are to be taken as illustrative embodiments only of the same,and that various changes in the details and arrangement of componentsand parts, as well as various procedural changes, may be resorted towithout departing from the spirit of the invention.

I claim:
 1. A method for removing cesium from a liquid metal circulatingthrough a primary cooling system of a nuclear reactor comprising:providing a primary carbon packing trap in said primary system,continually absorbing cesium from said coolant onto said carbon packingtrap at a relatively low temperature until said trap is substantiallysaturated with cesium, isolating said primary trap when saturated fromsaid primary system by disconnecting said trap therefrom and connectingsaid primary trap to a closed regeneration flow system having asecondary carbon packing trap, said closed regeneration flow systembeing completely isolated from said primary coolant system and having avolume substantially smaller than said primary coolant system,increasing the temperature of the coolant passing through said primarytrap, diffusing a portion of the cesium inventory on the surface of saidprimary trap further into the matrix thereof while dispersing a portionof said cesium inventory into said coolant circulating through saidregeneration flow system, reducing the temperature of said coolantpassing through said secondary trap, and physically and chemicallyabsorbing cesium from the coolant in said regeneration flow system ontosaid secondary carbon packing trap until cesium equilibrium between saidtraps is reached whereby the primary trap is reconditioned for furtherusage when reconnected to the primary system.
 2. A method according toclaim 1 wherein said coolant passing through said primary trap whenconnected to said primary system is cooled to a temperature ranging fromabout 170° to 230° C.
 3. A method according to claim 1 wherein saidcoolant passing through said primary trap when connected to saidregeneration flow system is heated to a temperature of about 500° C.while said coolant passing through said secondary trap is cooled to atemperature ranging from 170° to 230° C.
 4. In a primary coolant systemof a nuclear reactor having a liquid metal reactor coolant, a cesiumremoval system comprising: a primary carbon packing trap in said primarysystem for absorbing cesium from the coolant passing therethrough at arelatively low temperature, a closed regeneration flow system ofsubstantially smaller volume than said primary system having a secondarycarbon packing trap and normally disconnected from said primary system,means isolating said primary trap from said primary system when saidprimary trap is substantially saturated with cesium, means connectingsaid primary trap to said closed regeneration flow system, means forheating said coolant passing through said primary trap to diffuse aportion of the cesium inventory on the surface of said primary trapfurther into the matrix thereof while dispersing a portion of the cesiuminventory thereof into said regeneration flow system, means for coolingsaid coolant passing through said secondary trap for absorbing cesiumfrom the coolant in said regeneration flow system until cesiumequilibrium is reached between said primary and secondary traps wherebythe primary trap is reconditioned for further usage when reconnected tosaid primary system.
 5. A system according to claim 4, wherein thevolume of carbon packing in said secondary trap ranges from about 10 to50% of the volume of carbon packing in said primary trap.
 6. A systemaccording to claim 4, including a heat exchanger selectively coupled tosaid primary system and said regeneration flow system for assisting theheating means and cooling means in increasing and decreasing,respectively, the temperature of said coolant passing through saidprimary and secondary traps.